In human vision, light wavelength and intensity are perceived as color and brightness, respectively. Humans, however, lack the ability to perceive one fundamental physical property of light—its polarization. Information about light polarization can, however, be captured by image sensors and used to perform tasks such as contrast enhancement, material identification, and edge detection, etc. Image sensors used for polarization imaging may employ a CMOS or CCD image sensor with a polarization filter bonded to it or integrated monolithically.
Two prevalent architectures for sampling polarization intensity are “division-of-time” and “division-of-focal-plane” sensors. In division-of-time polarization imagers, a CMOS or CCD image sensor is coupled with a polarization filter designed such that it can be mechanically, electrically, or acousto-optically modulated to change the light transmission axis, allowing for different polarization intensities to be captured from frame to frame. More specifically, such division-of-time polarization imagers require switching polarization filters from frame to frame to capture 3 or more polarization frames to compute the angle and degree of polarization. A drawback of these sensors is that the frame rate is limited, and motion during imaging causes image blur and distortion. Division-of-focal-plane imagers, on the other hand, use a polarization filter array over the pixels such that each pixel samples the intensity of light transmitted through one polarization filter. In a similar manner to the interpolation done in color images, the missing polarization intensities at each pixel location can be recovered from neighboring pixels. In other words, such division-of-focal plane imagers use a mosaic of polarization filters and perform interpolation in a similar way to demosaicing in conventional color imagers to obtain three or more polarization channels for all pixels. This results in reduced spatial resolution when conventional image sensors are employed.
Another fundamental limitation of conventional polarization imagers is their inability to efficiently and simultaneously capture color and polarization images. A common approach for obtaining color and polarization information involves the modulation of the color filter over an array of polarization-sensitive pixels, to sequentially capture different color frames in addition to the polarization intensities [See K. P. Bishop, H. D. McIntire, M. P. Fetrow, and L. McMackin, “Multispectral polarimeter imaging in the visible to near IR,” Proc. SPIE 3699, 49-57 (1999)]. This method is limited by the need for electrical or mechanical modulation of the filters. Another method, disclosed in U.S. Pat. No. 8,411,146 to Twede, involves stacking a polarization filter array over a color filter array. In this method, color frames and near-infrared polarization frames are sequentially captured by modulating the light reaching the focal plane array. A more recent approach involves the use of three vertically stacked p-n junctions at different depths in silicon to sample the short, medium, and long wavelengths transmitted through the polarization filter [See M. Kulkarni and V. Gruev, “Integrated spectral-polarization imaging sensor with aluminum nanowire polarization filters,” Opt. Express 20, 22997-23012 (2012)]. Because a silicon coefficient decreases with wavelength, shorter wavelengths will be absorbed more in the top p-n junctions, while longer wavelengths will dominate the signal in the bottom p-n junction. This allows full color reproduction, but interpolation is required to obtain missing polarization intensities at each pixel location. The complexity of the readout circuitry as well as poor color separation of the three junctions reduces the appeal of this method.
Yet another challenge that polarization imagers have to contend with is the trend of decreasing pixel sizes in CMOS image sensors. As pixel sizes decrease, such sensors face the same limitations as conventional imagers, including reduced full-well capacity and dynamic range, and concomitantly reduced signal-to-noise ratio (SNR) and image quality. In polarization imagers, this translates into a reduction in the range of polarization information that can be resolved.